In recent years, the development of the sound device that can clearly transmit the audio sound to a user even under the noisy environment has been advanced. For instance, JP2541062B (Patent Document 1) proposes a technique for correcting sound-signal properties, in view of the auditory-sense properties of the human (or user), while appropriately corresponding to changes of an external noise.
FIG. 10 shows a block diagram illustrating the construction of a sound (or audio) playback unit 1 (i.e., the sound device) related to the above Patent Document 1. This sound playback unit 1 includes a sound signal source 2 adapted for producing sound signals, an external-sound capturing means 3 adapted for capturing a surrounding noise, an external-sound auditory-sense-property calculation means 4 adapted for analyzing each sound for each frequency band, and an equalizer 5 capable of amplifying the sound for each frequency band.
Namely, in the sound playback unit 1, a surrounding sound is first captured by the external-sound capturing means 3, and then analyzed by the external-sound auditory-sense-property calculation means 4. Subsequently, the equalizer 5 is controlled, corresponding to the result of the analysis, to allow each sound signal outputted from the sound signal source 2 to be amplified by an amplifier 6 for each different frequency band, and then outputted from a speaker 7.
More specifically, in this sound playback unit 1, the external-sound auditory-sense-property calculation means 4 can serve to extract a sound component masked by a noise component and exceeding a certain or reference audible limit curve from each sound signal that has been captured by the external-sound capturing means 3 and then compensated or corrected for the frequency properties thereof by a property-correction circuit 8. Thereafter, a comparison circuit 10 compares the so-extracted sound component with another sound component which also exceeds the audible limit curve and is separately extracted by a sound-signal auditory-sense-property calculation circuit 9. In this way, a damping ratio of the sound component due to the noise can be obtained. In this case, undue change of such a calculated damping ratio can be suppressed by a low-pass filter (or LPF) 11. Then, based on each result of such operations, the amplification due to the equalizer 5 can be properly controlled, thereby correcting the sound signal properties corresponding to the changes of the surrounding noise.
Namely, in the above sound playback unit 1 related to the related art, the external-sound auditory-sense-property calculation means 4 can serve to calculate an amplification factor of each sound signal for each different frequency band based on the reference audible limit curve. However, this amplification factor is obtained only for addition or subtraction for the sound volume based on the analysis and comparison between the inputted external sound relative to the sound signal to be outputted. In other words, this sound playback unit 1, as described in the above Patent Document 1, is intended only for controlling the increase or decrease of the sound volume in such a simple manner that may tend to cause a rather uncomfortable feeling to the auditory sense of the user. Besides, this device requires unduly much time for catching and analyzing the external sound.